This invention relates generally to a method and apparatus for determining a carrier frequency offset encountered in wireless systems and using this information for synchronization purposes, and in particular to determining a large carrier frequency offset using symbols having a cyclic prefix.
Wireless communications systems using radio-frequency (RF) signals for transmitting data are rapidly gaining popularity. These include both continuous data transmission systems, such as digital broadcast TV, as well as systems transmitting data at random times in bursts, e.g., wireless local area networks (WLANs)
In a typical RF data transmission system baseband data is transmitted by a transmitter which processes the baseband data and modulates it on a transmit carrier frequency fct to generate an RF signal. The RF signal is usually composed of groups of symbols called data frames. FIG. 1 illustrates a data frame 10 in the time domain. Frame 10 is composed of a number of consecutive data symbols 12A through 12M. Symbol 12N is shown in more detail to reveal its useful portion spanning a symbol interval Ts and its guard portion containing, e.g., a cyclic prefix, and spanning a guard interval Tg. Guard interval Tg precedes symbol interval Ts. Therefore, each symbol 12 has a total duration of Tg+Ts seconds.
A receiver receives data frame 10, demodulates symbols 12 and processes them to retrieve the transmitted baseband data. In order to properly perform this function the receiver has to achieve proper symbol timing and frequency synchronization with the transmitter. There are several aspects of synchronization that require careful attention for proper reception of data frame 10.
First, the receiver must determine the exact timing of the beginning of each symbol 12 within frame 10. If correct timing is not known, the receiver will not be able to reliably remove the cyclic prefixes and correctly isolate individual symbols 12 before performing further processing.
Second, the receiver has to also perform a generally more difficult task of determining and correcting for carrier frequency offset xcex94fc. Ideally, the receive carrier frequency fcr should exactly match the transmit carrier frequency fct. If this condition is not met the mis-match contributes to a non-zero carrier offset xcex94fc. Depending on the type of symbols transmitted, inability to correct for carrier offset may prevent the receiver from recognizing symbols 12. Orthogonal frequency division multiplexing (OFDM), although a robust technique for efficiently, transmitting data over a channel, is very susceptible to a non-zero carrier offset xcex94fc. The technique uses a plurality of sub-carrier frequencies within a channel bandwidth to transmit the data. These sub-carriers are arranged for optimal bandwidth efficiency compared to more conventional transmission approaches, such as frequency division multiplexing (FDM), which waste large portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). In case symbols 12 are generated by OFDM, the consequence of a carrier offset will be a loss of orthogonality between the OFDM sub-carriers and inter-carrier interference (ICI). This, in turn, will result in a high bit error rate (BER) in the recovered baseband data.
The third synchronization issue is of concern in OFDM communications. Specifically, the transmitter""s sample rate has to be synchronized to the receiver""s sample rate to eliminate sampling rate offset. Any mis-match between these two sampling rates results in an increased BER.
The transmission of data through a channel via an OFDM signal provides several advantages over more conventional transmission techniques. These advantages include:
a) Tolerance to multipath delay spread. This tolerance is due to the relatively long symbol interval Ts compared to the typical time duration of the channel impulse response. These long symbol intervals prevent inter-symbol interference (ISI).
b) Tolerance to frequency selective fading. By including redundancy in the OFDM signal, data encoded onto fading sub-carriers can be reconstructed from the data recovered from the other sub-carriers.
c) Efficient spectrum usage. Since OFDM sub-carriers are placed in very close proximity to one another without the need to leave unused frequency space between them, OFDM can efficiently fill a channel.
d) Simplified sub-channel equalization. OFDM shifts channel equalization from the time domain (as in single carrier transmission systems) to the frequency domain where a bank of simple one-tap equalizers can individually adjust for the phase and amplitude distortion of each sub-channel.
e) Good interference properties. It is possible to modify the OFDM spectrum to account for the distribution of power of an interfering signal. Also, it is possible to reduce out-of-band interference by avoiding the use of OFDM sub-carriers near the channel bandwidth edges.
Although OFDM exhibits these advantages, prior art implementations of OFDM also exhibit several difficulties and practical limitations. The most important difficulty with implementing OFDM transmission systems involves timing and frequency synchronization between the transmitter and the receiver, as discussed above.
Prior art solutions to obtaining proper timing and synchronization in RF transmission systems depend, among other factors, on the transmission technique, i.e., type of symbol keying. In simple systems appropriate phase lock loops (PLLs), or zero-crossing circuits can be used in the receiver for determining the transmit carrier frequency fct. In addition, or independently of these solutions, data frame 10 may include training symbols which are recognized by the receiver and used to achieve timing and synchronization.
Specifically, in the case of OFDM signals, several solutions have been proposed. In U.S. Pat. No. 5,444,697, Leung et al. suggest a technique for achieving timing synchronization of a receiver to an OFDM signal on a frame-by-frame basis. The method, however, requires that a plurality of the OFDM sub-carriers be reserved exclusively for data synchronization, thus reducing the number of sub-carriers used for encoding and transmitting data. Furthermore, Leung does not suggest a technique for correcting the carrier frequency offset or sampling rate offset. Finally, Leung""s technique requires a loop-back to determine the phase and amplitude of each sub-channel, thereby rendering the technique unsuitable for broadcast applications such as digital TV.
In U.S. Pat. No. 5,345,440, Gledhill et al. present a method for improved demodulation of OFDM signals in which the sub-carriers are modulated with values from a quadrature phase shift keying (QPSK) constellation. However, the disclosure does not teach a reliable way to estimate the symbol timing. Instead, assuming approximate timing is already known, it suggests taking a fast Fourier transform (FFT) of the OFDM signal samples and measuring the spread of the resulting data points to suggest the degree of timing synchronization. This technique, however, requires a very long time to synchronize to the OFDM signal since there is an FFT element in the timing synchronization loop. Also, their method for correcting for carrier frequency offset assumes that timing synchronization is already known. Furthermore, the achievable carrier offset acquisition range is limited to half a sub-channel bandwidth. This very limited range for carrier offset correction is insufficient for applications such as digital television where carrier frequency offsets are :likely to be as much as several tens of sub-carrier bandwidths. Finally, the disclosure does not teach a method for correcting for sampling rate offset.
In U.S. Pat. No. 5,313,169, Fouche et al. suggest a method for estimating and correcting for the carrier frequency offset and sampling rate offset of a receiver receiving an OFDM signal.
The method requires the inclusion of two additional pilot frequencies within the channel bandwidth. The success of this method is limited because these pilot carriers are susceptible to multipath fading. Furthermore, Fouche et al. do not suggest a reliable method for determining symbol timing. They discuss subtracting the cyclic prefix from each symbol and then trying to find where there is a cancellation, but such a cancellation will not occur in the presence of carrier frequency offset. Also, because their synchronization loop includes a computationally complex FFT, synchronization takes a long time. Additionally, because the method does not correct for carrier frequency offset before taking the FFTs, the method will suffer from inter-carrier interference between the sub-carriers, thus limiting its performance. Finally, the method also has a limited acquisition range for the carrier frequency offset estimation.
In xe2x80x9cA Technique for Orthogonal Frequency Division Multiplexing Frequency Offset Correction,xe2x80x9d IEEE Transactions on Communications, Vol. 42, No. 10, Oct. 1994, pp. 2908-14, and in xe2x80x9cSynchronization Algorithms for an OFDM System for Mobile Communications.xe2x80x9d ITG-Fachtagung 130, Munich, October 26-28, 1994, pp. 105-113, Moose and Classen, respectively discuss two techniques for OFDM synchronization. Both methods involve the repetition of at least one symbol within an OFDM data frame. Moose""s method does not suggest a way to determine symbol timing while Classen""s method requires searching for a cancellation of two identical symbols after correcting for the phase shift introduced by the carrier frequency offset. This technique requires the re-computation of a correction factor for every new set of samples and is, therefore, tremendously computationally complex. Furthermore, neither author suggests an effective technique for estimating carrier frequency offset greater than one half of a sub-channel bandwidth. Consequently, the methods would not be suitable to the reception of OFDM digital TV signals. Classen does suggest a trial-and-error method for estimating carrier frequency offsets greater than one half of a sub-channel bandwidth by searching in increments of 0.1 sub-channel bandwidths. Such a method, however, is very slow and computationally complex, especially for offsets of several sub-carrier bandwidths.
The prior art also suggests an approach to timing and synchronization for OFDM based on the cyclic prefix contained in the guard interval Tg. For example, in xe2x80x9cA New Frequency Detector for Orthogonal Multicarrier Transmission Techniquesxe2x80x9d, IEEE 45th Vehicular Technology Conference, 2:804-809, Jul. 25-28, 1995, F. Daffra et al. describe a method using a correlation with the cyclic prefix to find the carrier frequency offset if it is less than xc2xd of the sub-carrier spacing and the symbol timing is known. Unfortunately, this method is limited to an acquisition range of xc2x1xc2xd of the sub-channel spacing.
Another method of using the cyclic prefix is described by M. Sandell et al. in xe2x80x9cTiming and Frequency Synchronization in OFDM Systems Using the Cyclic Prefixxe2x80x9d, Proceedings IEEE International Symposium on Synchronization, pp. 16-19, Dec. 1995. Again, the estimate of the carrier offset using this technique is valid only if the offset can be guaranteed to be less than xc2xd of the sub-carrier spacing.
Additional prior art is also discussed by the inventors in xe2x80x9cLow-overhead, Low-complexity Burst Synchronization for OFDMxe2x80x9d, IEEE International Conference on Communications (ICC), 3:1301-1306, Jun. 23-27, 1996 and in U.S. Pat. No. 5,732,113.
Unfortunately, in many practical situations the carrier offset is large. In particular, when dealing with OFDM systems the offset can be significantly larger than xc2xd of the sub-carrier spacing. Thus, what is required is an approach which increases the carrier frequency acquisition range in RF communications systems without relying on training symbols. Such an approach would accommodate transmission of more data in the same bandwidth. Specifically, in application to OFDM systems an increased carrier frequency acquisition range would improve the efficiency of OFDM communications systems.
In view of the above short-comings of the prior art, it is an object of the present invention to provide a method and apparatus for determining large carrier frequency offsets in RF signals. In particular, in application to OFDM systems the method and apparatus of invention are designed to increase the carrier frequency acquisition range significantly beyond xc2x1xc2xd sub-carrier spacings and preferably to a significant fraction of the total bandwidth of the OFDM signal.
It is another object of the invention to provide improved carrier frequency offset determination for purposes of achieving rapid and efficient timing and synchronization of RF signals, and especially OFDM signals. In particular, the method of the invention provides that the carrier offset to be found without the use of training symbols. Synchronization information can be derived by taking samples from any window with a length of a few symbols without waiting for a training symbol, thus reducing delay.
It is a further object of the present invention to ensure that the method is easy to implement and robust. The apparatus of the invention should be simple in construction and straightforward to implement, especially in OFDM systems. Yet another object of the invention is to provide for the method and apparatus of the invention to support carrier frequency offset computation in continuous transmission systems such as digital TV, and systems transmitting data in random bursts, e.g., WLANS.
Still another object of the invention is to adapt the method and apparatus for use in OFDM systems employing phase-shift keying as well as amplitude and phase-shift keying of symbols.
These and other object and advantages of the invention will be better appreciated after reading the detailed specification.
These objects and advantages are attained by a method of determining an integral portion of a carrier offset xcex94fc of a signal transmitted from a transmitter at a transmit carrier frequency fct. The signal consists of at least two data symbols S1 and S2, each having a useful part preceded by a cyclic prefix. In the time domain the useful part occupies a symbol interval Ts and the cyclic prefix occupies a guard interval Tg.
In accordance with the method, the first and second data symbols S1 and S2 are received at a receive carrier frequency fct and the integral portion of the carrier offset xcex94fc between the receive carrier frequency fcr and the transmit carrier frequency fct is calculated in the form of an integral multiple of the inverse 1/Ts of the symbol interval.
In the preferred embodiment the method of the invention is applied to data symbols which are multiplexed by the orthogonal frequency division multiplexing (OFDM) technique. In this case signals S1 and S2 are constructed from sub-symbols ck which are modulated on corresponding sub-carrier frequencies fk and transmitted in corresponding sub-channels. It is important that sub-symbols ck belong to a 2m-ary constellation of complex values equally spaced in phase. For example, the 2m-ary constellation can be a phase-shift keyed (PSK) constellation such as BPSK, QPSK, DQPSK, 8-PSK, 8-DPSK, 16-PSK and 16-DPSK. Alternatively, the 2m-ary constellation can be an amplitude and phase-shift keyed (APSK) constellation such as of 16-APSK, 16-DAPSK, 64-APSK and 64-DAPSK.
The spacing of sub-carrier frequencies of the OFDM signal should be equal to 1/Ts, i.e., for OFDM signals the integral carrier offset xcex94fc is determined in terms of and integral number of sub-carrier spacings.
Whether the signal is an OFDM signal or a different type of signal, the cyclic prefix contains a tail portion of the useful part of the symbol. In other words, the last portion of the signal transmitted during symbol interval Ts is repeated during guard interval Tg.
The sampling of data symbols S1 and S2 should be performed such that a number N of samples is taken during the symbol interval Ts and a number G of samples is taken during the guard interval Tg. Both numbers N and G are integers and G is preferably an odd integer. This can be ensured, e.g., by adjusting the length of guard interval Tg Preferably, N is a power of 2.
Preferably, the carrier offset xcex94fc is calculated in two portions; the integral portion and a fractional portion. In the event of OFDM signals, both portions can be expressed interms of sub-carrier spacings as xcex94fc=(z+y)/Ts, where z is an integer and y is a fraction such that |y|xe2x89xa6xc2xd. In any event, the total carrier offset xcex94fc is typically used for blind synchronization of the receiver with the transmitter.
The apparatus or circuit of the invention carries out the above method with the aid of a receiver equipped with a computing unit which calculates the integral portion of the carrier offset xcex94fc by processing data symbols S1 and S2. The circuit also has a synchronizing circuit which uses the integral portion of the carrier offset xcex94fc in conjunction with the fractional portion, which is computed by a sub-circuit, for blind synchronization of the receiver with the transmitter.
The details of the invention are set forth in the detailed description with the aid of the attached drawing figures.